1. Field of the Invention
The present invention relates to a multi-heterodyne stage for a receiver or spectrum analyzer.
2. Description of the Prior Art
Multi-heterodyne stages having two, and in many cases even three successive intermediate-frequency stages, are known (MeinkeGrundlach, Taschenbuch der Hochfrequenztechnik (Handbook of High Frequency Technology), 5th edition, pp. Q12-Q22). In order to reduce crosstalk of a first mixer (local oscillator) to a second mixer in double- or multi-heterodyne stages, it is known to decouple a portion of performance of a first local oscillator and to add this portion of performance to a first intermediate-frequency signal in a compensatory sense, via an adder arranged in a first intermediate-frequency stage. This known compensation for crosstalk of the first local oscillator is effective in only a very narrow frequency region, since phase and amplitude of a compensation signal must exactly match that of the first intermediate frequency signal. For very small frequency changes, however, the compensation can even exacerbate the noise.
It is therefore an object of the present invention to expand and improve upon a multiheterodyne stage such that the above-described disadvantages are avoided, and avoid crosstalk over a broad band from the first local oscillator to the second mixer using an uncomplicated structure.
According to a preferred embodiment of the invention, a complete conditioning of the first intermediate-frequency signal is carried out symmetrically, so that corresponding transformers at the output of the first mixer and at the input of the second mixer are unnecessary. The first intermediate-frequency stage is thus very low-loss and has improved noise characteristics. By balancing both intermediate-frequency signals in both parallel branches of the first intermediate-frequency stage to exactly the same levels and opposite phases, which can also be carried out electronically during operation, a sensitivity for low received frequencies is optimized and crosstalk of the first local oscillator over a broadband is suppressed. This symmetry balancing improves also the intermodulation products of a direct order, since these respectively lie in phase at both mixer outputs and thereby make no contribution to the desired signal, but rather are suppressed by a symmetrical intermediate-frequency filter. Any intermodulation products of the third order produced in the two amplifiers are likewise reduced, since amplification is distributed over two individual amplifiers. As a result of the symmetrical feeding of the second mixer, a transformer is unnecessary here as well.
The invention finds application not only for the most-frequently used double heterodyne stages, but also for multi-heterodyne stages in which two or more such successive intermediate-frequency (IF) selection stages are connected in a cascade fashion.
All customary circuits which exhibit a symmetrical output or symmetrical input are suitable as mixers. Proven to be particularly advantageous for this are ordinary diode ring mixers (see for example Meinke-Grundlach, p. Q21), which enable a symmetrical tap, or a symmetrical signal feed, directly at diode diagonals, without additional transformers. Thus results an additional benefit of this invention, that it makes such transformers, previously necessary for diode ring mixers, unnecessary.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.